Numerous devices for detecting in particular the passage of at least one mobile body in at least one zone described by the space in which the said mobile body advances are already known (French A-2.195.812 and 2.593.761).
These devices comprise two separate parts each, namely an interrogation element and a responder, one of the two separate parts being connected with the mobile body, the other being considered fixed in the space in respect to which the mobile body moves.
The interrogation element itself comprises a transmitting and receiving element.
It is connected to conventional information processing circuits operating in accordance with the planned applications and the processing circuits are connected with an electric power supply.
One of the long-term problems in particular, which has been overcome today is to develop a passive responder thus called because it is supplied by a signal received from the interrogation element and which therefore is without direct electric power supply, whether by batteries or by connection with a distribution net.
In one of the known devices (French A-2.195.812), the interrogation element comprises two transmitters, oriented in the direction of the zone where the passage of the responder is monitored, one of which emits a high frequency signal in the form of electromagnetic waves, the other a very low frequency signal which, in this case, creates an electrostatic field and additionally a receiver for the signal processed by the responder from the signal received from the transmitters during their passage through the aforementioned zone afterwards immediately retransmitted by the said responder. The responder mainly comprises antenna means with which a non-linear component, such as a diode, is connected because of its ability to modify and in particular mix the signals received prior to their re-transmission by the responder in the form of a modulated carrier wave.
The responder then demodulates this signal and processes its very low frequency component.
Conceived for use in monitoring the passage from an exit of a warehouse of articles, in which a sensitive label constitutes the responder, this device calls up an interrogation element, the dimensions of which do not particularly fit the restrictions imposed by the limits of the original pattern on the use by transport means They are mainly sensitive to the presence of a responder in a very large space and therefore, for example, lack the required precision for resetting the position indicators, and they are mainly disturbed by the monitoring means of a parallel channel to the extent that they not supply any information regarding the direction of movement.
In another presently known device (French A-2,593,761), the interrogation element comprises an amplitude modulation means for a high-frequency signal of very high power (approximately 10 GHz and 200 mW) with the aid of a signal of medium frequency (approximately 200 kHz), which does or does not scramble the carrier wave in accordance with the rythm of the frequency of the wave produced by an oscillator, and transmission means of the said high-frequency carrier wave pre-modulated in amplitude in this way towards the location where the passage of a responder is monitored by its movement in respect to the interrogation element.
This interrogation element also comprises reception means of at least one high-frequency carrier wave overmodulated in amplitude by the responder which, at the time when it passes the location monitored by the interrogation element, receives the transmitted signal and distributes, with the aid of a circulator. One fraction is fed to a channel, where a circuit restores the clock signals and applies them to a delay line which will then dephase them to a certain value of less than the size of the pulses, and another fraction to a channel where the already modulated carrier wave will again be cut by the pulses of the delay line.
The carrier wave thus double-cut is immediately retransmitted by the responder towards the receiver, which demodulates and processes the signal by phase comparison.
With such devices it is known to individualize the responders for the purpose of identifying at least one mobile body and at least one location of its movement.
For this purpose, the delay line of the various responders has its own adapted time constant and the phase comparator of the interrogation element receives the correct references during each possible dephasing.
To be able to determine the direction of movement of the mobile body, it is also known to use a responder comprising two signal reception circuits, which are separated from each other in accordance in a direction parallel to the path of the mobile body.
An advantageous known reception circuit device of the responder is called symmetrical, i.e. placed at the same distance from both ends of the re-transmission antenna of the responder.
Taking into account the small space requirement required by the circuits, they are provided in the form of printed circuits.
The disadvantages of this known device lie in the disturbance of the signal originating from channels parallel to the one monitored and, above all, in the lack of accuracy in determinating the passage of the mobile body at right angles to the fixed location in its path, taking into account the exigencies of the means, in particular computer means, assigned to process the information received, for example for determining the speed and insuring the readjustment of the revolution sensors.